Some tedious debugging explains why Pygments.rb cannot popen4 the Python interpreter under Nginx: Pyhton lives in /usr/local/bin which is not on the path for the Rails process created by Nginx/Passenger. Many ways to fix that. In my case, I added something to the Rails initialization to make sure /usr/local/bin is always on the path for the Rails process.

Now is works like a charm - you're looking at the results in the above code blocks for this tech note.